Transparent graft copolymers based on acrylate soft phases

ABSTRACT

The invention relates to graft copolymers—based on non-cross-linked acrylate soft phases from which styrenic monomers are grafted—with a defined micro-structure, having a high transparency, toughness and weather resistance (UV-stability), a process for their preparation and their use, and also to polymer blends comprising said graft copolymers and styrenic polymers, and shaped articles produced therefrom and their use.

The invention relates to graft copolymers—based on non-cross-linkedacrylate soft phases—with a defined micro-structure, having a hightransparency, toughness and weather resistance (UV-stability), a processfor their preparation and their use. The invention further relates tomolding compositions comprising said graft copolymers and their use.

Currently available transparent and tough materials comprising a blendof a hard matrix (co-) polymer (e.g. GPPS, SAN, SMMA) and an impactmodifier (e.g. ASA) require a refractive index matching between said twocomponents. Further currently available transparent and tough materialsare specialty polymers such as styrene-butadiene block co-polymers (SBC)which combine the toughness of HIPS with the transparency of GPPS butrequire an anionic polymerization method to allow the production ofexactly defined molecular structures.

Document CN-A 103254374 discloses a tough transparent linearpolystyrene-b-poly-n-butylacrylate-b-polystyrene triblockcopolymercomprising a poly-n-butylacrylate soft block and styrene (co-)polymerhard blocks. The block copolymers are obtained by using a controlledemulsion polymerization (reversible addition-fragmentation chaintransfer free radical polymerization technology) in presence ofamphiphilic molecules as reversible addition-fragmentation chaintransfer agents.

Patent application PCT/EP2016/056344 deals with a process for thepreparation of cross-linked ABS or ASA graft copolymers having acore/shell-morphology and their use as impact modifiers in blends withstyrenic polymers. The cross-linked graft base is obtained bypolymerization of an alkyl acrylate or a diene in presence of lowamounts (at most 0.75 wt.-%) of a chain transfer agent (relative tomonomer content of graft base). Acrylate graft bases further contain atmost 10 wt.-% of a polyfunctional cross-linking monomer. The graftsheath content is in particular preferably from 20 to 50 wt.-%. Allexamples deal with cross-linked ABS graft copolymers. Transparentmaterials or even transparent ASA graft copolymers are not mentioned.

It is one object of the invention to provide transparent, tough andweather resistant, especially UV-resistant, polymers and a process fortheir preparation that is simple to carry out on an industrial scale andwhich does not require any special polymerization techniques such ascontrolled radical or ionic polymerization methods. A further object ofthe invention is to provide transparent, tough and weather resistantmolding compositions comprising said polymer and a hard matrixthermoplastic polymer with no need for a refractive index matching ofsaid polymer components.

Subject of the invention is a graft copolymer B, built up from:

-   -   (B1) 15 to 45 wt.-%, preferably 20 to 40 wt.-%, based on graft        copolymer B, of a non-cross-linked graft substrate polymer B1        having a glass transition temperature T_(G) below 25° C. (DSC,        heating rate: 5K /min), consisting of polymerized units derived        from monomers B11, B12 and optionally B13:        -   (B11) from 95 to 99.5 wt.-%, based on the total weight of            B11, B12 and B13, of at least one C₁-C₁₀-alkyl acrylate;        -   (B12) from 0.5 to 5 wt.-%, based on the total weight of B11,            B12 and B13, of at least one bifunctional, crosslinking            monomer which contains two copolymerizable double bonds            which are not conjugated in 1,3 positions; and        -   (B13) from 0 to 4.5 wt.-%, based on the total weight of B11,            B12 and B13, of at least one other copolymerizable,            monoethylenically unsaturated monomer; and    -   (B2) 55 to 85 wt.-%, preferably 60 to 80 wt.-%, based on graft        copolymer B, of at least one polymer B2 having a glass        transition temperature T_(G) above 25° C., grafted (in the form        of branches) from the graft substrate polymer (B1), consisting        of polymerized units derived from monomers B21 and optional        comonomers B22:        -   (B21) from 65 to 100 wt.-% of at least one vinylaromatic            monomer and/or of at least one C₁-C₈-alkyl-(meth)acrylate,            preferably of at least one vinylaromatic monomer or its            mixture with at least one C₁-C₈-alkyl-(meth)acrylate; and        -   (B22) from 0 to 35 wt.-% of at least one other            monofunctional comonomer;    -   where components B1 and B2 give 100 wt.-% in total, and wherein    -   the graft substrate polymer B1 has a gel content (non-soluble        fraction in toluene) below 5 wt.-%, preferably below 3 wt.-%,        more preferably below 1 wt.-%, based on the total amount of B1;        and    -   the second double bond of the bifunctional, crosslinking monomer        B12 is the active site from which polymerized units derived from        monomer B21 and optionally comonomer B22 are grafted.

Wt.-% means % by weight. The amounts of components B11, B12 and B13 give100 wt.-% in total.

The glass transition temperature Tg is determined by DSC based on DIN ENISO 11357-2:2014-07 (heating rate: 5 K/min).

Graft Copolymer B

The graft copolymers B according to the invention are highly transparenthaving generally a transmittance of at least 75% and a haze coefficientbelow 10%, preferably below 5%. The transmission is determined accordingto ASTM D1003 and the haze is determined according to ASTM D1003-95.

The term “non-cross-linked graft substrate polymer B1” as usedhereinbefore means that the gel content (non-soluble fraction from theisolated polymer B1, solvent: toluene) of the graft substrate polymer B1is below 5 wt.-%, based on the total amount of B1.

The second double bond and active site of the bifunctional, crosslinkingmonomer B12 is a non-(meth)acrylic double bond in case monomer B12 is amonomer—other than di(meth)acrylates—which has only one (meth)acrylicdouble bond (e.g. AMA and DCPA).

The graft copolymer B comprises or consists of a soft phase and a hardphase. The non-cross-linked graft substrate polymer B1 (=backbonepolymer) constitutes the soft phase and provides elasticity for highelongation and impact resistance of the material. Polymer B2 constitutesthe hard phase and is responsible for the material toughness, necessaryfor the high mechanical strength.

Moreover, the graft copolymers B have a specific, preferably lamellar,micro-structure in difference to the common core/shell-morphology of theimpact modifier particles of ASA-graft copolymers according to the priorart. To ensure excellent transparency of the graft copolymer B, thedomain size of the graft copolymer B is below the wave-length of visiblelight. Due to incompatibility of the different phases graft copolymer Bshows a microphase separation, this results in a periodic nanostructure,where the distance of the different phases is below the wavelength ofvisible light (similar to a lattice constant).

Preferably the number average molar mass M_(n) (measured by SECanalysis, see experimental part) of the graft substrate polymer B1 is inthe range of from 20.000 to 50.000 g/mol, more preferably 20.000 to40.000 g/mol.

The grafted polymerized units derived from monomer B21 and optionallycomonomer

B22 constituting polymer B2 (=hard phase) form, preferably linear,branches which branches may have the same or preferably a differentlength. Graft copolymers B having an unsymmetrical architecture areoften preferred. The short branches of such preferred unsymmetricalgraft copolymers B ensure compatibility in blends with a differentpolymer A, in particular a hard phase matrix polymer, and the longbranch(es) provide high mechanical strength.

The graft copolymers B according to the invention can be used on itsown, optionally in combination with additives and/or auxiliaries C, as ahigh performance polymer (high impact resistance and transparency) or asa blend with one or more further polymers A.

Further subject of the invention are molding compositions comprisinggraft copolymer B and optionally additives and/or auxiliaries C.Preferred molding compositions comprise graft copolymer B and additivesand/or auxiliaries C. Said additives and/or auxiliaries C are commonlyknown in the art.

Said molding compositions can further comprise at least onethermoplastic polymer A.

The amount of said additives and/or auxiliaries C is generally 0 to 10wt.-%, preferably 0.1 to 5 wt.-%, more preferably 0.2 to 2 wt.-%, basedon the overall molding composition comprising (consisting of) componentsB and C, and optionally component A. If additives and/or auxiliaries Care present in said composition, their minimum fraction is customarily0.1 wt %.

The sum of the components B and C, and optionally component A, makes 100wt %.

For the preparation of the inventive molding composition said additivesand/or auxiliaries C can be mixed with graft copolymer B or polymer Aalone, or can be added to their mixture.

Graft substrate polymer B1

The graft substrate polymer B1 has a glass transition temperature T_(G)below 25° C. (DSC, heating rate: 5 K/min), preferably below 0° C., mostpreferably below −25° C.

According to the invention the graft substrate polymer B1 consists ofpolymerized units derived from monomers B11, B12 and optionally B13:

(B11) from 95 to 99.5 wt.-%, preferably 96 to 99 wt.-%, more preferably97 to 98.5 wt.-% based on the total weight of B11, B12 and B13, of atleast one C₁-C₁₀-alkylacrylate;

(B12) from 0.5 to 5% wt.-%, preferably 1 to 4 wt.-%, more preferably 1.5to 3 wt.-% based on the total weight of B11, B12 and B13, of at leastone bifunctional, crosslinking monomer which contains twocopolymerizable double bonds which are not conjugated in 1,3 positions;and

(B13) from 0 to 4.5 wt.-%, preferably 0 to 3 wt.-%, more preferably 0 to1.5 wt.-% based on the total weight of B11, B12 and B13, of at least oneother copolymerizable, monoethylenically unsaturated monomer.

The amounts of monomers B11, B12 and B13 give 100 wt.-% in total.

Particularly suitable C₁-C₁₀-alkyl acrylates (B11) are methyl acrylate,ethyl acrylate, propyl acrylate, n-butyl acrylate, n-pentyl acrylate,n-hexyl acrylate, n-heptyl acrylate, n-octyl acrylate, 2-ethyl-hexylacrylate, n-nonyl acrylate and n-decyl acrylate, and also mixtures ofthese, particularly preferably ethyl acrylate, 2-ethylhexyl acrylate,n-butyl acrylate or mixtures of these, and very particularly preferablyn-butyl acrylate.

Examples of copolymerizable, bifunctional crosslinking monomers (B12)suitable for crosslinking are ethylene glycol diacrylate, polyethyleneglycol diacrylate, ethylene glycol dimethacrylate, butanedioldiacrylate, butane-diol dimethacrylate, hexanediol diacrylate,hexanediol dimethacrylate, divinylbenzene, diallyl maleate, diallylfumarate, diallyl phthalate, tricyclodecenyl acrylate,dihydrodicyclopentadienyl acrylate, allyl acrylate, allyl methacrylate(AMA) and dicyclo-pentadienyl acrylate (DCPA), preferably AMA and DCPA.

Other examples which may be mentioned of copolymerizable,monoethylenically unsaturated monomers (B13) are butadiene, isoprene;vinylaromatic monomers, such as styrene or α-methyl styrene;methacrylonitrile, acrylonitrile; acrylic acid, methacrylic acid,dicarboxylic acids, such as maleic acid and fumaric acid, and alsoanhydrides of these, such as maleic anhydride; nitrogen-functionalmonomers, such as dimethylaminomethyl acrylate, dimethylaminoethylacrylate, vinylimidazole, vinylpyrrolidine, vinylcaprolactam,vinylcarbazole, vinylaniline, acrylamide; C₁-C₄-alkyl methacrylates,such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate,isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate andsec-butyl methacrylate; aromatic or araliphatic (meth)acrylates, such asphenyl acrylate, phenyl methacrylate, 2-phenylethyl acrylate,2-phenylethyl methacrylate, benzyl methacrylate, benzyl acrylate,2-phenoxyethyl methacrylate and 2-phenoxyethyl acrylate; unsaturatedethers, such as vinyl methyl ether, and also mixtures of these monomers.

Preferred monomers (B13) are styrene and/or α-methyl styrene.

Preferably optional monomer B13 is not present.

The graft substrate polymer B1 contains a small amount ofcopolymerizable, bifunctional crosslinking monomer (B12) that acts asactive site (binding site) for the graft copolymerization. Thebifunctional co-monomer is selected to have one double bond reactingduring the synthesis of the backbone and one double bond reacting duringthe grafting polymerization. In order to ensure phase compatibility thegraft substrate polymer B1 is grafted from without further modification.

Polymer B2

Polymer B2 has a glass transition temperature T_(G) above 25° C.,preferably above 50° C., most preferably above 100° C. Polymer B2consists of polymerized units derived from monomers B21 and optionalcomonomers B22:

-   -   (B21) from 65 to 100 wt.-%, often 65 to 85 wt.-%, of at least        one vinylaromatic monomer and/or of at least one        C₁-C₈-alkyl-(meth)acrylate, preferably of at least one        vinylaromatic monomer or its mixture with at least one        C₁-C₈-alkyl-(meth)acrylate; and    -   (B22) from 0 to 35 wt.-%, often 15 to 35 wt.-%, of at least one        monofunctional comonomer B22.

The amounts of monomers B21 and B22 give 100 wt.-% in total.

The vinylaromatic monomer (B21) is preferably styrene, α-methyl-styreneor ring-C₁-C₈-alkyl-alkylated styrenes, such as p-methylstyrene ortert-butylstyrene, particularly preferably styrene or α-methylstyrene,most preferably styrene.

According to the invention, the C₁-C₈-alkyl (meth)acrylates (B21)preferably used are methyl methacrylate (MMA), ethyl methacrylate, n- orisopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate,sec-butyl methacrylate, tert-butyl methacrylate, pentyl methacrylate,hexyl methacrylate, heptyl methacrylate, octyl methacrylate or2-ethylhexyl methacrylate, particularly preferably methyl methacrylate,or mixtures of these monomers, methyl acrylate (MA), ethyl acrylate,propyl acrylate, n-butyl acrylate, isobutyl acrylate, sec-butylacrylate, tert-butyl acrylate, pentyl acrylate, hexyl acrylate, heptylacrylate, octyl acrylate or 2-ethylhexyl acrylate, particularlypreferably n-butyl acrylate, or else a mixture of these monomers withone another or preferably with the afore-mentioned methacrylates, wherethe amount of the acrylates in the graft shell is preferablysubordinate.

Preferably used as monomer (B21) are styrene, α-methylstyrene and/ormethyl methacrylate, more preferably styrene, α-methylstyrene, or amixture of styrene or α-methylstyrene with methyl methacrylate.

In case component (B21) is a mixture of at least one, preferably one,vinylaromatic monomer and of at least one, preferably one,C₁-C₈-alkyl-(meth)acrylate monomer, said mixture preferably consists of65 to 85 wt.-% vinylaromatic monomer B21, in particular styrene orα-methylstyrene, and 15 to 35 wt.-% C₁-C₈-alkyl-(meth)acrylate monomer,in particular methyl methacrylate.

Possible monofunctional comonomers (B22) are monomers selected from thegroup consisting of methacrylonitrile, acrylonitrile, N—C₁-C₈-alkyl-,N—C₅-C₈-cycloalkyl- and N—C₆-C₁₀-aryl-substituted maleimides, such asN-methyl-, N-phenyl-, N-dimethylphenyl- and N-cyclohexylmaleimide, andmaleic anhydride. Maleic anhydride and in particular acrylonitrile arepreferred.

It is preferable for polymer B2 to have been built up from styrene, orfrom a mixture consisting of 65 to 85 wt.-% styrene and 15 to 35 wt.-%acrylonitrile, maleic anhydride or methyl methacrylate, in particularacrylonitrile.

Process for the Preparation of Graft Copolymer B

A further subject of the invention is a process for the preparation ofgraft copolymer B as described above, which comprises (consistsessentially of) the following steps:

-   -   (i)free radical aqueous emulsion polymerization of monomers B11,        B12 and optionally B13 in presence of an initiator PI-1; and    -   (ii) grafting monomer B21 and optional comonomer B22 from graft        substrate polymer B1 obtained in step (i) by free radical        emulsion or solution polymerization in presence of an initiator        PI-2;    -   where the initiators PI-1 and PI-2 can be the same or different        compounds, and wherein    -   step i) is performed in presence of 0.85 to 2 wt.-%, preferably        0.90 to 2.0 wt.-%, more preferably 0.95 to 1.80 wt.-%, most        preferably 0.95 to 1.60 wt.-%, of at least one chain transfer        agent, based on the total amount of monomers B11, B12 and B13;        and    -   in step ii)    -   the amount of the initiator PI-2 used is 0.1 to 3 wt.-%,        preferably 0.5 to 2 wt. %, relative to the total content of        monomers B21 and B22; and    -   monomer B21 and, if present, comonomer B22 are fed continuously        to the reaction mixture within 7 to 20 hours, preferably 8 to 18        hours, more preferably 9 to 15 hours at a temperature of from 50        to 100° C.

In said process monomers B11, B12, B13, B21 and B22 and their amountsare the same as hereinbefore defined for graft copolymer B.

In step (i) of the process of the invention, the graft substrate polymerB1 is prepared, by polymerizing the monomers B11, B12 and optionallyB13, in an aqueous emulsion, in presence of an initiator PI-1 at atemperature range from 20 to 100° C., preferably from 50 to 80° C., morepreferably 50 to 65° C. Use may be made of the usual emulsifiers andinitiators for emulsion polymerization.

Usual emulsifiers are such as alkali metal alkyl-or alkylarylsulfonates,alkyl sulfates, fatty alcohol sulfonates, salts of higher fatty acidshaving from 10 to 30 carbon atoms, sulfosuccinates, ether sulfonates, orresin soaps. Preference is given to the use of the sodium or potassiumalkylsulfonates, or of salts of fatty acids having from 10 to 18 carbonatoms.

The usual amounts of emulsifiers may be used. Advantageous amounts ofemulsifiers are from 0.3 to 5% by weight, in particular from 1 to 2% byweight, based on the monomers used in preparing the graft substratepolymer B1.

The latex of graft substrate polymer B1 is preferably prepared usingsufficient water to give the finished latex a solid content of from 20to 50% by weight.

Preferred polymerization initiators (PI) are free-radical generators,for example peroxides, preferably peroxosulfates, such as potassiumperoxodisulfate, and azo compounds, such as azodiisobutyronitrile.However, it is also possible to use redox systems, in particular thosebased on hydroperoxides, such as cumene hydroperoxide. Saidpolymerization initiators (PI) can be used independently aspolymerization initiator PI-1 in step (i) or as polymerization initiatorPI-2 in step (ii) of the inventive process.

The amount of the polymerization initiator(s) PI-1 used in step (i) isusually from 0.1 to 1% by weight, based on the content of the monomersused in preparing the graft substrate polymer B1.

It is essential for the process according to the invention that theemulsion polymerization in step (i) is performed in presence of at leastone, preferably one, chain transfer agent, such as ethylhexylthioglycolate, tert-dodecyl mercaptan, terpinols or dimericα-methylstyrene, in an amount of 0.85 to 2 wt.-%, preferably 0.90 to 2.0wt.-%, more preferably 0.95 to 1.80 wt.-%, most preferably 0.95 to 1.60wt.-%, based on the total amount of monomers B11, B12 and B13. The useof tert-dodecyl mercaptan (TDM) is in particular preferred.

The chain transfer agent in step i) is preferably added stepwise in twoor more portions, preferably 2 to 5 portions, more preferably 3portions. It is further preferred that said portions are equal portions,based on the total amount of the chain transfer agent. It is furthermorepreferred, that in step i) the chain transfer agent is added stepwise intwo or more portions, preferably 2 to 5 portions, more preferably 3portions within 2 to 5 hours, in particular 3 to 4 hours.

To maintain a constant pH, preferably from 6 to 9, buffer substances maybe used as polymerization auxiliaries, for example Na₂HPO₄/NaH₂PO₄,sodium hydrogencarbonate or sodium carbonate. The usual amounts of thebuffer substances are used, and further details in this connection aretherefore unnecessary.

The precise polymerization conditions, in particular the type, method ofaddition and amount of the emulsifier, are generally determinedindividually within the ranges given above in such a way as to give thenon-cross-linked graft substrate polymer B1 with a gel content(non-soluble fraction in toluene) below 5 wt.-%, preferably below 3wt.-%, more preferably below 1 wt.-%, based on the total amount of B1.

It is also in principle possible to prepare the graft substrate polymerB1 by a process other than emulsion polymerization, e.g. by bulk (mass)or solution polymerization, and then to emulsify the resultant polymers.The latter step is only one option. Usually in case of a solutionpolymerization, all steps (preparation and chemically modification ofthe graft substrate polymer and its grafting) are performed in solution.Microsuspension polymerization is also suitable, preferably usingoil-soluble initiators, such as lauroyl peroxide or tert-butylperpivalate. The processes for this are known.

Graft polymerization step (ii) of the process of the invention, can beconducted by emulsion or solution polymerization, preferably in aqueousemulsion in the presence of an aqueous emulsion of the graft substratepolymer B1. In the latter case the graft co-polymerization can becarried out in a system which is the same as that used for thepolymerization of the graft substrate polymer B1 with addition offurther initiator (=initiator PI-2) and, if required, of furtheremulsifier. These do not have to be the same as the emulsifiers orinitiators used for preparing the graft substrate polymer B1. Preferablywater soluble initiators such as peroxosulfates, in particular potassiumperoxodisulfate, are used as initiator PI-2.

According to the invention the amount of the initiator PI-2 used inprocess step (ii) is usually from 0.1 to 3 wt.-%, preferably 0.5 to 2wt.-%, most preferably 0.5 to 1.5 wt.-%, based on the content of themonomers used in preparing polymer B2.

The emulsifier, the initiator PI-2 and the polymerization auxiliariesmay each be charged on their own or in a mixture to the graft substratepolymer B1. Any of the possible combinations of, on the one hand,charging and feeding and, on the other hand, initiator, emulsifier andpolymerization auxiliaries may be used. Preferred embodiments are thoseknown to the skilled worker.

According to the invention, in step (ii) of the process monomer B21 and,if present, comonomer B22 are fed (dosed) to the reaction mixture within7 to 20 hours, preferably 8 to 18 hours, more preferably 9 to 15 hoursat a temperature of from 50 to 100° C., preferably 55 to 70° C. In step(ii) optionally the initiator PI-2 can also be fed (dosed) to thereaction mixture within said time range.

Preferably the initiator is added in one portion at the beginning ofstep (ii).

Monomers B21 and B22 (which in polymerized form constitute polymer B2)to be grafted from the active sites (=second double bond of thebifunctional, crosslinking monomer B12) of graft substrate polymer B1are added continuously to the reaction mixture during thepolymerization.

The graft polymerization step (ii) of the process according to theinvention is usually conducted within 7 to 20 hours at a temperature offrom 50 to 100° C., preferably 55 to 70° C.

If run in emulsion, the solid content of graft copolymer B is between 30and 50 wt.-%.

Alternatively, after isolation of the product obtained in step (i) in aknown manner such as precipitation and drying, step (ii) of the processof the invention can be conducted by solution polymerization in anorganic solvent such as toluene using an oil soluble initiator PI-2 suchas benzoyl peroxide (BPO) or azobisisobutyronitrile (AIBN). The amountof said oil-soluble initiator Pi-2 is also in the range as defined forinitiator PI-2 above. In case of a solution polymerization step (ii) isalso conducted at a temperature and time range as defined above.

If run in solution, the solid content of graft copolymer B is between 15and 50 wt.-%.

The aqueous dispersion of the graft polymers B—obtained by emulsionpolymerization step ii)—is worked up in a manner known per se.Customarily, first of all, the graft polymer b is precipitated in thedispersion, by addition of precipitating salt solutions (such as calciumchloride, magnesium sulfate, alum) or acids (such as acetic acid,hydrochloric acid or sulfuric acid), for example, or else by shearprecipitation or freezing (freeze coagulation). The aqueous phase can beremoved in a customary way, for instance by sieving, filtering,decanting or centrifuging. This prior separation of the dispersion waterproduces water-moist graft copolymers B having a residual water contentof up to 60 wt %, based on B, in which case the residual water, forexample, may adhere externally to the graft copolymer B and may also beincluded within it.

The graft polymer B can subsequently, as and when required, be driedfurther in a known way, for example, using hot air or by means of apneumatic dryer. It is also possible to work up the dispersion by spraydrying.

In case that grafting step ii) has been conducted as solutionpolymerization the polymer can be isolated from the solution in mannerknown to a skilled person. For example the solvent can be removed by anevaporation extruder or the polymer B can be precipitated, e.g. withmethanol, and filtered off. After isolating polymer B it might be driedin an oven (between 50 and 80° C.) with or without vacuum.

The inventive process does not require any special polymerizationtechniques (such as controlled radical or anionic polymerizationmethods) to obtain a copolymer having a defined micro-structure butrather relies on simple and economically free radical polymerizationtechniques.

Due to its economically attractive production, graft copolymer B isdesigned to replace specialty polymer produced by anionicpolymerization.

A further subject of the invention is graft copolymer B as describedabove obtained by the inventive process as hereinbefore described.

Polymer Blends

A further subject of the invention is a molding composition—alsoreferred to as polymer blend—comprising at least one graft copolymer Bas described above and at least one thermoplastic polymer A having aglass transition temperature above 25° C., preferably above 50° C., mostpreferably above 100° C.

Preferably thermoplastic polymer A is at least one polymer selected fromthe group consisting of: standard polystyrene (GPPS, homopolystyrene),styrene-acrylonitrile copolymers (SAN), α-methylstyrene-acrylonitrilecopolymers (AMSAN), styrene-maleic anhydride copolymers (SMSA),styrene-N-phenylmaleimide copolymers (SNPMI), styrene-methylmethacrylate copolymers (SMMA), styrene-acrylonitrile-maleic anhydridecopolymers, styrene-acrylonitrile-phenylmaleimide copolymers,α-methylstyrene-acrylonitrile-methyl methacrylate copolymers,α-methylstyrene-acrylonitrile-tert-butyl methacrylate copolymers,styrene-acrylonitrile-tert-butyl methacrylate copolymers, andpoly(meth)acrylates (e.g. polymethylmethacrylate (PMMA)). Morepreferably thermoplastic polymer A is selected from standard polystyrene(GPPS, homopolystyrene), styrene-acrylonitrile copolymers (SAN),α-methylstyrene-acrylonitrile copolymers (AMSAN), styrene-maleicanhydride copolymers (SMSA) or styrene-methyl methacrylate copolymers(SMMA). In particular preferred are GPPS, SAN and/or SMMA.

The weight average molar mass Mw of the thermoplastic polymer A is inthe range of 150 000 to 360 000 g/mol, determined by GPC (solvent:tetrahydrofuran, polystyrene as polymer standard) with UV detection.

Suitable standard polystyrene is produced by the method of anionic orradical polymerization. The nonuniformity of the polymer, which can beinfluenced by the polymerization process, is of minor importance here.Preference is given to standard polystyrene whose toluene-solublefraction has an average molecular weight Mw of 150 000 to 300 000 g/mol,more preferably 150 000 to 270 000 g/mol, and which is optionallyfurther furnished with additives, such as, for example, mineral oil(e.g., white oil), stabilizer, antistats, flame retardants or waxes.

SAN copolymers and α-methylstyrene-acrylonitrile copolymers (AMSAN) usedas polymer A in accordance with the invention contain generally 18 to 35wt %, preferably 20 to 32 wt %, more preferably 22 to 30 wt % ofacrylonitrile (AN), and 82 to 65 wt %, preferably 80 to 68 wt %, morepreferably 78 to 70 wt % of styrene (S) or α-methylstyrene (AMS), wherethe sum of styrene or α-methylstyrene and acrylonitrile makes 100 wt %.

The SAN and AMSAN copolymers used generally have an average molar massMw of 150 000 to 350 000 g/mol, preferably 150 000 to 300 000 g/mol,more preferably 150 000 to 250 000 g/mol, and very preferably 150 000 to200 000 g/mol.

Suitable SAN copolymers are commercial SAN copolymers such as Luran®from Ineos Styrolution (Frankfurt) for example.

SMMA copolymers used as polymer A in accordance with the inventioncontain generally 18 to 50 wt %, preferably 20 to 30 wt %, of methylmethacrylate (MMA), and 50 to 82 wt %, preferably 80 to 70 wt %, ofstyrene, where the sum of styrene and MMA makes 100 wt %.

SMSA copolymers used as polymer A in accordance with the inventioncontain generally 10 to 40 wt %, preferably 20 to 30 wt %, of maleicanhydride (MAN), and 60 to 90 wt %, preferably 80 to 70 wt %, ofstyrene, where the sum of styrene and MAN, makes 100 wt %.

Mentioned in particular as poly(meth)acrylates may be polymethylmethacrylate (PMMA) and also copolymers based on methyl methacrylatewith up to 40 wt % of further copolymerizable monomers, of the kindavailable, for example, under the designations Lucryl® from Lucite orPlexiglas® from Evonik (Germany).

The polymers A are obtained in a known way by bulk, solution,suspension, precipitation or emulsion polymerization, with bulk andsolution polymerization being preferred. Details of these processes aredescribed for example in Kunststoffhandbuch, edited by R. Vieweg and G.Daumiller, volume 4 “Polystyrol”, Carl-Hanser-Verlag Munich 1996, p. 104ff, and also in “Modern Styrenic Polymers: Polystyrenes and StyrenicCopolymers” (Eds., J. Scheirs, D. Priddy, Wiley, Chichester, UK, (2003),pages 27 to 29) and in GB-A 1472195.

In said polymer blends polymer A functions as a hard matrix polymer andgraft copolymer B is used as a modifier, enhancing the properties(impact resistance, transparency, weatherability, mechanical strength)of said hard matrix polymers.

Due to the small domain size of the graft copolymers B even thecombination with a polymer A with a different refractive index yields atransparent blend. In contrast to prior art materials there is no needfor a refractive index matching between the two components.

Furthermore, the inventive polymer blend may optionally compriseadditives and/or auxiliaries C. The amount of the additives and/orauxiliaries C is generally 0 to 10 wt.-%, preferably 0.1 to 5 wt.-%,more preferably 0.2 to 2 wt.-%, based on the overall molding compositioncomprising (or consisting of) components B, A and C. If additives and/orauxiliaries C are present in the molding composition, their minimumfraction is customarily 0.1 wt %.

The sum of the components A and B and optionally C present in theinventive polymer blend makes 100 wt %.

Preferably the inventive molding composition comprises (or consists of):

5 to 60 wt.-%, preferably 10 to 55 wt.-%, more preferably 10 to 55 wt.-%of at least one graft copolymer B,

30 to 95 wt.-%, preferably 40 to 89.9 wt.-%, more preferably 43 to 89.8wt.-% of at least one thermoplastic polymer A, and

0 to 10 wt.-%, preferably 0.1 to 5 wt.-%, more preferably 0.2 to 2 wt.-%of additives and/or auxiliaries C,

wherein the sum of the amounts of components A, B and, if present C,makes 100 wt.-%.

Preferred according to the invention is a molding composition (polymerblend) consisting of at least one graft copolymer B and at least onethermoplastic polymer A.

Furthermore preferred according to the invention is a moldingcomposition (polymer blend) consisting of at least one graft copolymerB, at least one thermoplastic polymer A and additives and/or auxiliariesC.

The molding composition of the invention may optionally comprisecommonly known additives and/or auxiliaries C such as stabilizers,oxidation retarders, agents to counter thermal decomposition anddecomposition due to ultraviolet light, lubricants and mold releaseagents, colorants such as dyes and pigments, fibrous and pulverulentfillers and reinforcing agents, nucleating agents, plasticizers, etc..The fraction thereof being in general not more than 10 wt.-%, preferablynot more than 5 wt.-%, more preferably not more than 2 wt.-%. Ingeneral—within the afore-mentioned ranges—the amounts of said additivesand/or auxiliaries C are chosen such that a certain translucency will beretained.

Examples of oxidation retarders and heat stabilizers are halides of themetals from group I of the periodic table, examples being sodium,potassium and/or lithium halides, optionally in combination withcopper(I) halides, e.g., chlorides, bromides, iodides, stericallyhindered phenols, hydroquinones, different substituted representativesof these groups, and mixtures thereof, in concentrations of up to 1wt.-%, based on the weight of the thermoplastic molding composition.

UV stabilizers, used generally in amounts of up to 2 wt.-%, based on themolding composition, include various substituted resorcinols,salicylates, benzotriazoles, and benzophenones.

Furthermore, organic dyes may be added, such as nigrosine, pigments suchas titanium dioxide, phthalocyanines, ultramarine blue, and carbon blackas colorants, and also fibrous and pulverulent fillers and reinforcingagents. Examples of the latter are carbon fibers, glass fibers,amorphous silica, calcium silicate (wollastonite), aluminum silicate,magnesium carbonate, kaolin, chalk, powdered quartz, mica, and feldspar.The fraction of such fillers and colorants is generally up to 10 wt.-%,preferably up to 5 wt.-%.

Examples of nucleating agents that can be used are talc, calciumchloride, sodium phenylphosphinate, aluminum oxide, silicon dioxide, andnylon 22.

Examples of lubricants and mold release agents, which can be used ingeneral in amounts up to 1 wt.-%, are long-chain fatty acids such asstearic acid or behenic acid, their salts (e.g., Ca or Zn stearate) oresters (e.g., stearyl stearate or pentaerythrityl tetrastearate), andalso amide derivatives (e.g., ethylenebisstearylamide). For betterprocessing, mineral-based antiblocking agents may be added in amounts upto 0.1 wt.-% to the molding compositions of the invention. Examplesinclude amorphous or crystalline silica, calcium carbonate, or aluminumsilicate.

Processing aids which can be used are, for example, mineral oil,preferably medical white oil, in amounts up to 5 wt.-%, preferably up to2 wt.-%.

Examples of plasticizers include dioctyl phthalate, dibenzyl phthalate,butyl benzyl phthalate, hydrocarbon oils, N-(n-butyl)benzenesulfonamide,and o- and p-tolylethylsulfonamide.

For further improving the resistance to inflammation, it is possible toadd all of the flame retardants known for the thermoplastics inquestion, more particularly those flame retardants based on phosphoruscompounds and/or on red phosphorus itself.

The polymer blends of the invention may be produced from components Aand B (and optionally further additives and/or auxiliaries C) by allknown methods.

The graft polymers B are prepared by free radical polymerization, asalready described above. The obtained graft copolymers B mayeither—preferably in case that grafting step (ii) has been conducted assolution polymerization—be mixed directly with components A and/or C, orit may be worked up beforehand. The latter approach is preferred.

The graft copolymers B are mixed with the polymer A and, where present,with the further components C in a mixing apparatus, producing asubstantially liquid-melt polymer mixture.

“Substantially liquid-melt” means that the polymer mixture, as well asthe predominant liquid-melt (softened) fraction, may further comprise acertain fraction of solid constituents, examples being unmelted fillersand reinforcing material such as glass fibers, metal flakes, or elseunmelted pigments, colorants, etc. “Liquid-melt” means that the polymermixture is at least of low fluidity, therefore having softened at leastto an extent that it has plastic properties.

Mixing apparatuses used are those known to the skilled person.Components A and B, and—where included—C may be mixed, for example, byjoint extrusion, kneading, or rolling, the aforementioned componentsnecessarily having been isolated from the aqueous dispersion or from thesolution obtained in the polymerization. Where one or more components inthe form of an aqueous dispersion or of a nonaqueous solution are mixedin, the water and/or the solvent is removed from the mixing apparatus,preferably an extruder, via a degassing unit.

Examples of mixing apparatus for implementing the method includesdiscontinuously operating, heated internal kneading devices with orwithout RAM, continuously operating kneaders, such as continuousinternal kneaders, screw kneaders with axially oscillating screws,Banbury kneaders, furthermore extruders, and also roll mills, mixingroll mills with heated rollers, and calenders.

A preferred mixing apparatus used is an extruder. Particularly suitablefor melt extrusion are, for example, single-screw or twin-screwextruders. A twin-screw extruder is preferred.

In some cases the mechanical energy introduced by the mixing apparatusin the course of mixing is enough to cause the mixture to melt, meaningthat the mixing apparatus does not have to be heated. Otherwise, themixing apparatus is generally heated. The temperature is guided by thechemical and physical properties of components A and B and—whenpresent—C, and should be selected such as to result in a substantiallyliquid-melt polymer mixture. On the other hand, the temperature is notto be unnecessarily high, in order to prevent thermal damage of thepolymer mixture. The mechanical energy introduced may, however, also behigh enough that the mixing apparatus may even require cooling. Mixingapparatus is operated customarily at 160 to 400° C., preferably 180 to300° C.

In one preferred embodiment, the mixing of the aforementioned componentstakes place in an extruder, with the graft copolymer B being separatedbeforehand from the dispersion water. As a result of this prior removalof the dispersion water, water-moist graft copolymers B are obtainedwhich have a residual water content of up to 60 wt.-%, based on B. Theresidual water present may then be removed in vapor form as describedabove via degassing facilities in the extruder. With particularpreference, however, the residual water in the extruder is not removedsolely as steam; instead, a part of the residual water is removedmechanically in the extruder and leaves the extruder in the liquidphase. In the case of this so-called squeeze method (EP-B 0 993 476, pp.13-16), the same extruder is supplied with the polymer A and—wherepresent—components C, meaning that the product of the method extruded isthe completed polymer blend.

However, the components of the polymer blend of the present inventionmay also be dry-blended, for example in a tumble blender. Saiddry-blended polymer blends can be used directly in a process for theproduction of shaped articles, e.g. by extrusion, injection molding orblow molding.

Further said polymer blends can be extruded and the extruded moldingcomposition can be used in a process for the production of shapedarticles, e.g. by extrusion, injection molding or blow molding.

The molding composition of the present invention can be used for theproduction of shaped articles or moldings, e.g. foils.

Furthermore, the present invention relates to a molding or shapedarticle comprising (or made of) a molding composition as describedabove. The molding or shaped article can be used in various fields ofapplications of transparent, in particular highly transparent, polymerarticles. The moldings or shaped articles can be e.g. a food container,display racks, crisper trays, and components of toys.

Furthermore, the present invention relates to the use of a moldingcomposition as described above for the production of household items,electronic components, household equipment, garden equipment,medical-technology equipment, motor-vehicle components, and bodyworkparts. In particular the polymer blend as described above can be usedfor the production of a food container. In particular the polymer blendas described above can be used for the production of highly transparentobjects (e.g. foils).

The following examples and claims further illustrate the invention.

EXAMPLES

Materials

THF, and methanol, are supplied by VWR chemicals as chromatographygrades and are used without any further purification. Aluminum oxide(activated, basic, Brockmann Grade I, 58 angstroms) was purchased fromAlfa Aesar and was used as received. The monomer inhibitor is removedfrom the used monomers by adsorptive filtration using an aluminum oxidecolumn.

Methods for Characterization

SEC analysis is performed with an Agilent 1100 Series HPLC system,equipped with UV (254 nm) detector and RI detector (Agilent 1100series). The following column set from Agilent is used: PL gel 5 μmguard column (50×7.5 mm), 2 PL gel 5 μm Mixed-C columns (300×7.5 mm).The analysis is performed using THF as solvent for the sampledissolution as well as for elution solvent. The samples are dissolved at1 mg/mL concentration and the flow rate is fixed at 1 mL/min. Apolystyrene standard set from Polymer Standard Service PSS is used forcalibration.

HPLC analysis is performed with an Agilent 1100 Series HPLC system,using a Nucleosil 100-5 OH (250×4.6 mm) column. An evaporative lightscattering detector PL-ELS 2100 from Agilent is used.

1H NMR analysis is recorded on a Bruker advance 300 NMR spectrometer(frequency at 300.38 MHz) in CDCl₃ at 298.1 K. Chemical shifts arereported in 6 units (ppm) relative to the remaining resonances of thesolvent at 7.26 ppm.

NMR analysis is used to determine the hard phase content in the graftcopolymer B by integration of the corresponding hard and soft phasepeaks.

The molar percentage of hard phase is calculated as follows.

${{mol}.\mspace{14mu} \%_{{hard}\mspace{14mu} {phase}}} = \frac{{Int}_{({{hard}\mspace{14mu} {phase}\mspace{14mu} {peak}})}}{\frac{{Int}_{({{soft}\mspace{14mu} {phase}\mspace{14mu} {peak}})}}{{member}\mspace{14mu} H} + \frac{{Int}_{({{hard}\mspace{14mu} {phase}\mspace{14mu} {peak}})}}{{member}\mspace{14mu} H}}$${{wt}.\mspace{14mu} \%_{{hard}\mspace{14mu} {phase}}} = \frac{\frac{{mol}.\mspace{14mu} \%_{{hard}\mspace{14mu} {phase}}}{M_{{hard}\mspace{14mu} {phase}}}}{\frac{1 - {{mol}.\mspace{14mu} \%_{{hard}\mspace{14mu} {phase}}}}{M_{{soft}\mspace{14mu} {phase}}} + \frac{{mol}.\mspace{14mu} \%_{{hard}\mspace{14mu} {phase}}}{M_{{soft}\mspace{14mu} {phase}}}}$

The analysis is used to quantify the number of olefinic groups remainingon the grafted backbone after graft copolymerization. The grafting yieldcan therefore be defined as:

${\eta ({grafting})} = \frac{{{mol}.\mspace{14mu} \%}\mspace{14mu} {of}\mspace{14mu} {olefinic}\mspace{14mu} {after}\mspace{14mu} {grafting}}{{{mol}.\mspace{14mu} \%}\mspace{14mu} {of}\mspace{14mu} {olefinic}\mspace{14mu} {before}\mspace{14mu} {grafting}}$

Film casting from solvent: The dried graft copolymer B is dissolved intoluene (3 wt. % solution). The solution is cast in a petri dish and thesolvent is allowed to evaporate at room temperature for minimum 24 h.The film is removed from the petri dish by dipping. Then, the film isdried in a vacuum oven for a minimum of 24 h. The films are used formechanical testing, optical measurements and TEM analysis.

Mechanical properties Tensile strength, elongation and E-modulus of thesamples are measured using a Zwick Roell Z 2.5 device.

Optical properties: Transmittance, clarity and haze coefficients aredetermined by using a Gardner Haze hard plus. The transmission isdetermined according to ASTM D1003 and the haze is determined accordingto ASTM D1003-95.

TABLE 1 Preparation of Graft Copolymer B graft step (ii) substratemonomer polymer B1 dosing used from Examples Polymer Type time (h)example 1 PBA-co- graft substrate polymer — DCPA B1 synthesis, step (i)2 PBA-co- graft substrate polymer — AMA B1 synthesis, step (i) 3 PBA-co-Grafting 10 1 DCPA-g- copolymerization, PS step (ii) 4 PBA-co- Grafting10 2 AMA-g- copolymerization, PS step (ii)

Example 1: Emulsion Polymerization of Polybutylacrylate-co-DCPA (2 wt.-%DCPA)

The reaction vessel is charged with 151.52 g of demineralized water,1.525 g of a 40 wt.-% sodium alkylsulfonate (C14 to C18) solution inwater and 0.23 g sodium bicarbonate and subsequently evacuated and purgewith nitrogen. After heating the reaction vessel to 59° C., 0.18 gpotassium persulfate is added to the reaction mixture. A mixture of 2.04g DCPA (dihydrodicyclopentadienyl acrylate) and 100 g butyl acrylate isadded within 210 min under constant stirring. After 30 min, 90 min and150 min a portion of each 0.33 g tert-Dodecylmercaptan is added. Thepost polymerization time is 60 min at a temperature of 61° C. For thepreparation a scaling factor of 35.6 was used.

An average particle diameter of 80 nm was determined by turbidity forthe latex particles. The gel content in toluene is below 2 wt.-%.

An aqueous solution containing 1 wt. % of MgSO₄ and 0.06 wt. % of H₂SO₄was prepared and heated up to 50° C. The prepared latex was poured in aminimum of 5 folds of the aqueous acidic solution, heated up to 90° C.to coagulate. The polymer was then filtered and dried in a vacuum ovenfor 24 hours at 50° C.

Example 2: Emulsion Polymerization ofPolybutylacrylate-co-Allylmethacrylate (2.39 wt.-% AMA)

The reaction vessel is charged with 151.52 g of demineralized water,1.525 g of a 40 wt.-% sodium alkylsulfonate (C14 to C18) solution inwater and 0.23 g sodium bicarbonate and subsequently evacuated and purgewith nitrogen. After heating the reaction vessel to 59° C., 0.18 gpotassium persulfate is added to the reaction mixture. A mixture of 2.45g AMA (allyl methacrylate) and 100 g butyl acrylate is added within 210min under constant stirring. After 30 min, 90 min and 150 min a portionof each 0.33 g tert-Dodecylmercaptan is added. The post polymerizationtime is 60 min at a temperature of 61° C. For the preparation a scalingfactor of 20 was used. A diameter of 87 nm was determined by turbidityfor the latex particles. The Gel content in Toluene is below 1 wt.-%.

An aqueous solution containing 1 wt. % of MgSO₄ and 0.06 wt. % of H₂SO₄was prepared and heated up to 50° C. The prepared latex was poured in aminimum of 5 folds of the aqueous acidic solution, heated up to 90° C.to coagulate. The polymer was then filtered and dried in a vacuum ovenfor 24 hours at 50° C.

Example 3: Grafting of Styrene from Polybutylacrylate-co-DCPA

80g of the emulsion from example 1 (31.44g of PBA-co-DCPA) is charged ina 500 mL reactor equipped with an impeller stirrer. A solution of water(187.24 g) with sodium dodecyl sulfate (0.75 g) and NaHCO₃ (0.088 g) isadded to the mixture and stirred at room temperature at 200rpm. Themixture is heated up to 60° C. and potassium persulfate is added to themixture (0.5 g). 125.76 g of Styrene is added to the mixture in thecourse of 10 hours, after which the mixture is kept at 60° C. for 10more hours.

The produced emulsion is mixed with 500 mL of deionized water understirring. 50 mL of a 20 wt.-% aqueous solution of Mg₂SO₄ is added to themixture for precipitation. The polymer is isolated by filtration andwashed with water. The product is then dried in a convection oven for 24hours at 60° C. and in a vacuum oven at 60° C. for a further 24 hours.

Example 4: Grafting of Styrene from Polybutylacrylate-co-AMA

80 g of the emulsion from example 2 (31.44 g of PBA-co-AMA) is chargedin a 500 mL reactor equipped with an impeller stirrer. A solution ofwater (187.24 g) with sodium dodecyl sulfate (0.75 g) and NaHCO₃ (0.088g) is added to the mixture and stirred at room temperature at 200rpm.The mixture is heated up to 60° C. and potassium persulfate is added tothe mixture (0.5 g). 125.76 g of Styrene is added to the mixture in thecourse of 10 hours, after which the mixture is kept at 60° C. for 10more hours.

The produced emulsion is mixed with 500 mL of deionized water understirring. 50 mL of a 20 wt.-% aqueous solution of Mg₂SO₄ is added to themixture for precipitation. The polymer is isolated by filtration andwashed with water. The product is then dried in a convection oven for 24hours at 60° C. and in a vacuum oven at 60° C. for a further 24 hours.

Example 5

A molding composition is prepared from 50% by weight of the polymer fromexample 1 and 50% by weight of commercial SAN-copolymer (Luran®).

1-19. (canceled)
 20. A graft copolymer B, built up from: (B1) 15 to 45wt.-%, preferably 20 to 40 wt.-%, based on graft copolymer B, of anon-cross-linked graft substrate polymer B1 having a glass transitiontemperature T_(G) below 25° C. (DSC, heating rate: 5K/min), consistingof polymerized units derived from monomers B11, B12, and optionally B13:(B11) from 95 to 99.5 wt.-%, based on the total weight of B11, B12, andB13, of at least one C₁-C₁₀-alkyl acrylate; (B12) from 0.5 to 5 wt.-%,based on the total weight of B11, B12, and B13, of at least onebifunctional, crosslinking monomer which contains two copolymerizabledouble bonds which are not conjugated in 1,3 positions; and (B13) from 0to 4.5 wt.-%, based on the total weight of B11, B12, and B13, of atleast one other copolymerizable, monoethylenically unsaturated monomer;and (B2) 55 to 85 wt.-%, preferably 60 to 80 wt.-%, based on graftcopolymer B, of at least one polymer B2 having a glass transitiontemperature T_(G) above 25° C., grafted from the graft substrate polymer(B1), consisting of polymerized units derived from monomers B21 andoptional comonomers B22: (B21) from 65 to 100 wt.-% of at least onevinylaromatic monomer or its mixture with at least oneC₁-C₈-alkyl-(meth)acrylate; and (B22) from 0 to 35 wt.-% of at least oneother monofunctional comonomer; where components B1 and B2 give 100wt.-% in total, and wherein the graft substrate polymer B1 has a gelcontent (non-soluble fraction in toluene) below 5 wt.-%, preferablybelow 3 wt.-%, based on the total amount of B1; and the second doublebond of the bifunctional, crosslinking monomer B12 is the active sitefrom which polymerized units derived from monomer B21 and optionallycomonomer B22 are grafted.
 21. The graft copolymer B according to claim20 having a transmittance T of at least 75% (determined according toASTM D1003) and a haze coefficient below 10% (determined according toASTM D1003-95).
 22. The graft copolymer B according to claim 20, whereinmonomer B11 is ethyl acrylate, 2-ethylhexyl acrylate, n-butyl acrylate,or mixtures thereof.
 23. The graft copolymer B according to claim 20,wherein the bifunctional, crosslinking monomer B12 is allyl methacrylate(AMA) or dicyclo-pentadienyl acrylate (DCPA).
 24. The graft copolymer Bto claim 20, wherein monomer B21 is styrene, α-methylstyrene, or amixture of styrene or α-methylstyrene with methyl methacrylate.
 25. Thegraft copolymer B according to claim 20, wherein comonomer B22 is maleicanhydride or acrylonitrile.
 26. The graft copolymer B according to claim20, wherein polymer B2 has been built up from styrene, or from a mixtureconsisting of 65 to 85 wt.-% styrene and 15 to 35 wt.-% acrylonitrile,maleic anhydride, or methyl methacrylate.
 27. The graft copolymer Baccording to claim 20, wherein monomer B13 is not present.
 28. The graftcopolymer B according to claim 20, built up from: (B1) 20 to 40 wt.-% ofgraft substrate polymer B1 and (B2) 60 to 80 wt.-% of polymer B2.
 29. Aprocess for the preparation of the graft copolymer B according to claim20, which comprises the following steps: (i) free radical aqueousemulsion polymerization of monomers B11, B12, and optionally B13 inpresence of an initiator PI-1; and (ii) grafting monomer B21 andoptional comonomer B22 from graft substrate polymer B1 obtained in step(i) by free radical emulsion or solution polymerization in presence ofan initiator PI-2; where the initiators PI-1 and PI-2 can be the same ordifferent compounds, and wherein step i) is performed in presence of0.85 to 2 wt.-% of at least one chain transfer agent, based on the totalamount of monomers B11, B12, and B13; and in step ii): the amount of theinitiator PI-2 is 0.1 to 3 wt.-%, preferably 0.5 to 2 wt. %, relative tothe total content of monomers B21 and B22; and monomer B21 and, ifpresent, comonomer B22 are fed continuously to the reaction mixturewithin 7 to 20 hours at a temperature of from 50 to 100° C.
 30. Theprocess according to claim 29, wherein in step i) the chain transferagent is added stepwise in two or more portions.
 31. The processaccording to claim 29, wherein in step ii) the monomer, and optionalinitiator, feed is within 9 to 15 hours.
 32. A graft copolymer Bobtained by the process according to claim
 29. 33. A molding compositioncomprising the graft copolymer B according to claim 20, and optionallyadditives and/or auxiliaries C.
 34. The molding composition according toclaim 33, further comprising at least one thermoplastic polymer A havinga glass transition temperature above 25° C.
 35. The molding compositionaccording to claim 34, wherein the thermoplastic polymer A is selectedfrom standard polystyrene (GPPS, homopolystyrene), styrene-acrylonitrilecopolymers (SAN), α-methylstyrene-acrylonitrile copolymers (AMSAN),styrene-maleic anhydride copolymers (SMSA), and styrene-methylmethacrylate copolymers (SMMA).
 36. The molding composition according toclaim 34, comprising: 5 to 60 wt.-% of at least one graft copolymer B,30 to 95 wt.-% of at least one thermoplastic polymer A, and 0 to 10wt.-% of additives and/or auxiliaries C, wherein the sum of the amountsof components A, B, and, if present, C, makes 100 wt.-%.
 37. A shapedarticle, such as a food container, a display rack, a crisper tray, a toycomponent, and a foil, comprising the molding composition according toclaim
 33. 38. The molding composition according to claim 33 for theproduction of a household item, an electronic component, householdequipment, garden equipment, medical-technology equipment, amotor-vehicle component, and a bodywork part.